[REQUEST]: Make Type for backendOptions

[partouf]

Is your feature request related to a problem? Please describe

backendOptions has been roaming around too freely around the code, and it's currently hard to figure out what can end up in there

We also may want to keep a list of the key names that are allowed so that we can sanitize it properly (this should also be done with filters)

Describe the solution you'd like

Shared type that can be maintained and enforced

Describe alternatives you've considered

Additional context

Not applicable